1. Field of the Invention
The present invention relates to a molybdenum base alloy having high-temperature strength and a method of producing the same. More particularly, the present invention relates to a molybdenum base alloy having high-temperature strength, which contains a given amount of at least one of zirconium and hafnium, a given amount of carbon, and further contains not more than given amounts of oxygen and nitrogen, and a method of producing the same.
2. Description of the Prior Art
Metallic molybdenum has excellent high-temperature strength, and it is known that molybdenum base alloy containing carbon and alloy elements, such as Ti, Zr, Hf and the like, has better high-temperature strength than that of metallic molybdenum. The reason why the molybdenum base alloy has better high-temperature strength than that of metallic molybdenum is considered to be as follows. The carbide formed by the addition of the above described alloy elements to metallic molybdenum suppresses the grain boundary migration or causes precipitation hardening to raise the recrystallization temperature, that is, softening temperature, and the decrease of the strength of the alloy is small even at high temperature.
However, in order to raise the above described recrystallization temperature, a relatively large amount of alloy elements has hitherto been added to molybdenum, and for example, Zr is added to molybdenum so that the resulting molybdenum base alloy contains at least 0.03% by weight of Zr, and Hf is added to molybdenum so that the alloy contains at least 0.05% by weight of Hf. Addition of such an amount of Zr or Hf to molybdenum raises the recrystallization temperature of the resulting molybdenum base alloy, but the malleability, ductility and workability thereof are poor.
Molybdenum or molybdenum base alloy has hitherto been mainly produced by the powder metallurgy process or vacuum arc melting process.
In general, in the powder metallurgy process, a powdery raw material mixture of molybdenum and alloy elements or the compounds thereof is press molded, and the molded article is heated and sintered at about 1,700.degree. C. The resulting alloy is subjected to swaging or other working to be made into a malleable material. In the powder metallurgy process, molybdenum is alloyed by the diffusion or penetration of particles of alloy elements or the compound thereof into molybdenum, and therefore it is very difficult to obtain molybdenum base alloys having a homogenous microscopic composition. Accordingly, when a mixture of metallic molybdenum powder and finely divided compounds, such as oxide, carbide and nitride, which are hardly soluble in the molybdenum matrix, is press molded and the molded article is heated and sintered, a molybdenum base alloy containing foreign substance phase particles dispersed therein can be obtained. The foreign substance phase particles prevent the grain boundary migration at the recrystallization treatment of the alloy and suppresses the recrystallization of the molybdenum matrix. Therefore, in the powder metallurgy process, even metal oxides, which are chemically stabler than molybdenum oxide and can maintain the solid phase at the sintering temperature or working temperature of molybdenum, can be used as foreign substance phase particles. This phenomenon is the same as in the case of metal nitride and carbide.
The vacuum arc melting process also is used in the melting of molybdenum or molybdenum base alloy. High-strength molybdenum base alloy is an alloy containing Ti together with at least one of Zr and Hf.
However, among the molybdenum base alloys produced by the powder metallurgy process or vacuum arc melting process, the higher tensile strength an alloy has, a higher temperature is necessary in the hot working of the alloy and therefore the workability thereof is poorer.